I'm sure there must be an easy way to do this, but given the Fourier transform of an isotropic filter kernel, $\hat{f}(\mathbf{u}) = \mathcal{F}f(\mathbf{z})$, can one calculate the value of the kernel at $\mathbf{z} = 0$?

$\begingroup$In the other direction (from the "spatial" domain to the "frequency" domain) it is more intuitive: The component at f=0 frequency (DC level) is what you get when you average the signal at the "spatial" domain.$\endgroup$
– nimrodmMay 15 '13 at 8:20

1

$\begingroup$@nimrodm oh yes of course, great descriptive explanation. I forgot the inv FT is just the FT with a 180 deg phase change.$\endgroup$
– geometrikalMay 15 '13 at 8:32